1. Field of the Invention
The present invention relates to an optical scanner which irradiates a light beam to a scanned surface of an image support to form an electrostatic latent image on the image support surface, and an optical-path adjustment method which is used for the optical scanner, and an image forming apparatus which includes the optical scanner provided therein.
2. Description of the Related Art
The electrophotographic image forming apparatuses, such as copiers, printers, facsimiles, plotters, etc. are known. Each of these image forming apparatuses is provided with an optical scanner which irradiates a light beam to a scanned surface of an image support to form the electrostatic latent image on the image support surface.
These image forming apparatuses may be classified into four major types which follow.
(1) The monochrome image forming apparatus in which the light beam which is emitted by a light source is irradiated to an image support (for example, a photoconductive drum) to form an electrostatic latent image on the image support surface, the latent image formed on the image support is converted into the visible image by the developer (for example, the toner of black), the visible image is transferred to a recording material (for example, paper) by using a transferring unit, and after the transferring the image is fixed to the recording material by using a fixing unit.
(2) The color image forming apparatus in which the light beam which is emitted by a light source is irradiated to an image support (for example, a photoconductive drum) to form an electrostatic latent image on the image support surface, the latent image formed on the image support is converted into the visible images by two or more developers of different colors (for example, the toners of yellow, magenta, cyan, and black), the visible images of the different colors are combined by using a primary transferring unit, the multi-color image after the primary transferring is transferred to a recording material (for example, paper) by using a secondary transferring unit, and after the secondary transferring the multi-color image is fixed to the recording material by using a fixing unit.
(3) The color image forming apparatus in which the light beams which are emitted by plural light sources are irradiated to plural image supports (for example, photoconductive drums) to form electrostatic latent images on the image support surfaces, the latent images formed on the image supports are converted into the visible images by two or more developers of different colors (for example, the toners of yellow, magenta, cyan, and black), the visible images of the different colors are combined and transferred to a recording material (for example, paper) by transporting the recording material to the respective transfer units of the image supports by means of a transfer conveyance belt, and after the transferring the multi-color image is fixed to the recording material by using a fixing unit.
(4) The color image forming apparatus in which the light beams which are emitted by plural light sources are irradiated to plural image supports (for example, photoconductive drums) to form electrostatic latent images on the image support surfaces, the latent images formed on the image supports are converted into the visible images by two or more developers of different colors (for example, the toners of yellow, magenta, cyan, and black), the visible images of the different colors are combined by using a primary transferring unit, the multi-color image after the primary transferring is transferred to a recording material (for example, paper) by transporting the recording material to the respective transfer units of the image supports by means of a transfer conveyance belt and a secondary transferring unit, and after the secondary transferring the multi-color image is fixed to the recording material by using a fixing unit.
In the optical scanner provided for the optical writing in each of the image forming apparatuses, the following deviating conditions may take place:                A. The resist deviation in the sub-scanning direction (or the counterpart) (FIG. 13A)        B. The scanning line inclination in the sub-scanning direction (or the counterpart) (FIG. 13B)        C. The scanning line bending in the sub-scanning direction (or the counterpart) (FIG. 13C)        D. The resist deviation in the main scanning direction (or the counterpart) (FIG. 13D)        E. The scale-factor deviation in the main scanning direction (or the counterpart) (FIG. 13E)        F. The non-uniformity of the scanning speed in the main scanning direction (or the counterpart) (FIG. 13F)        
The main scanning direction is the direction in which the optical scanner performs the optical writing to the image support, and the sub-scanning direction is the direction in which the recording material is moved. Hereinafter, it is supposed that the sub-scanning direction is substantially perpendicular to the main scanning direction.
Although the optical writing direction and the movement direction of the recording material are at right angles mechanically, the recording material is moved in the sub-scanning direction while it is scanned in the main scanning direction. Strictly speaking, the image in the main scanning direction and the image in the sub-scanning direction are not perpendicular to each other.
Moreover, the intermediate position on the optical path prior to entering the recording material does not correspond to the main scanning direction and the sub-scanning direction. Hereinafter, the direction that corresponds to the main scanning at the position of the optical writing on the optical path is called the main scanning correspondence direction, and the direction that corresponds to the sub-scanning at the position of the optical writing on the optical path is called the sub-scanning correspondence direction.
The resist deviation in the sub-scanning direction of the item A above is a shifting of the scanning line from the ideal scanning line in parallel with the sub-scanning direction as shown in FIG. 13A. This phenomenon arises due to changes of the sub-scanning-direction performance of the optical elements and the geometric precision of each optical element, caused by thermal expansion of each optical element.
The scanning line inclination in the sub-scanning direction of the item B above is an inclination of the scanning line from the ideal scanning line in the sub-scanning direction as shown in FIG. 13B. This phenomenon arises due to changes of the sub-scanning-direction performance of the optical elements and the geometric precision of each optical element.
The scanning line bending in the sub-scanning direction of the item C above is a bending of the scanning line to the ideal scanning line in the sub-scanning direction as shown in FIG. 13C. This phenomenon arises due to changes of the sub-scanning-direction performance of the optical elements, the geometric precision of each optical element, and the deformation of each optical element.
The resist deviation in the main scanning direction of the item D above is a shifting of the scanning start position between respective scanning lines as shown in FIG. 13D. This phenomenon arises due to a small variation of the laser diode wavelength during the multi-beam scanning, the difference of the field inclination of the multiple surfaces of the polygon mirror, the difference of the quantity of light in several modes of image formation, etc.
The scale-factor deviation in the main scanning direction of the item E above is a phenomenon in which the lengthes of the scanning lines differ in the main scanning direction from the ideal length as shown in FIG. 13E. This phenomenon arises due to changes of the sub-scanning-direction performance of the optical elements and the geometry precision of each optical element, caused by thermal expansion of each optical element. Moreover, it is generated by a small variation of the laser diode wavelength in the multi-beam scanning or the like.
The non-uniformity of the scanning speed in the main scanning direction of the item F above is a phenomenon in which the optical writing of the light beam is not suitably performed at the ideal scanning position because the scanning speed in the main scanning direction varies microscopically. This phenomenon arises due to changes of the main-scanning-direction performance of the optical elements and the geometry precision of each optical element, caused by thermal expansion of each optical element.
In order to obviate the sub-scanning-direction resist deviation of the item A above, the correction of the position relation of the optical scanner and the paper end is performed by adjusting the emission start timing of the light source with respect to the sub-scanning direction. There is no necessity for high precision adjustment of the optical scanner. It is adequate that the optical scanner adjustment is performed in a range that does not cause interference with the main part component members by heat deformation etc.
However, the color image forming apparatuses of the types (3) and (4) above need the detection units for setting the scanning start timing of the optical scanners of the respective colors. These detection units detect the light beam or the pixel position.
To obviate the scanning-line inclination in the sub-scanning direction of the item B above, the necessary scanning line inclination characteristics are obtained with the precision of the optical scanner component parts in the image forming apparatus which does not need the positional precision so much as in the monochrome image forming apparatus of the type (1) above.
Moreover, in the image forming apparatus which needs the comparatively high position precision, parallelism adjustment is carried out in the attachment portions with the optical scanner, and the inclination characteristics of the reproduced image are obtained.
In the case of the optical scanner provided in the image forming apparatus of the types (3) and (4) above, inclination adjustment of the optical scanner and parallelism adjustment with the optical scanner are carried out according to the scanning inclination adjustment mechanism which changes the position of the reflection mirror around the axis that is perpendicular to the main scanning direction and at right angles to the reflection surface of the mirror.
To obviate the scanning line bending in the sub-scanning direction of the item C above, the necessary scanning line bending characteristics are obtained with the precision of the optical scanner component parts in the image forming apparatus which does not need the positional precision so much as in the image forming apparatuses of the types (1) and (2) with the single optical path.
In the case of the optical scanner provided in the image forming apparatus which needs the comparatively high positional precision, for example, the types (3) and (4) with the multiple optical paths, the central part of the optical element with the function to correct the position of the scanning line in the sub-scanning direction is deformed, and thereby the scanning line bending in the sub-scanning direction is adjusted.
To obviate the resist deviation in the main scanning direction of the item D above, fundamentally, the optical detection unit including the photodiodes is provided in the optical scanner or the main part of the optical scanner at the location that is outside the image formation area. The scanning start position of the main scanning direction is determined by forming image information by the optical scanning based on the time which indicates the passing of the light beam detected by the optical detection unit.
To obviate the deviation of the scanning start position due to variations of the field inclinations of the plural surfaces of the polygon mirror, the component part precision is raised to a level which does not appear in the reproduced image.
Moreover, to obviate the difference of the quantity of light in the several image formation modes and the small variation of LD wavelength by the multi-beam scanning, the scanning start timing is controlled according to the quantity of light in each image formation mode.
To obviate the scale-factor deviation in the main scanning direction of the item E above, the necessary scale-factor characteristics are obtained with the precision including the thermal expansion of the optical scanner component parts in the image forming apparatus which does not need the positional precision so much as in the monochrome image forming apparatus of the type (1).
Moreover, in the image forming apparatus which needs the comparatively high positional precision, two separate optical detection units including the photodiodes are provided in the optical scanner or the main part of the optical scanner at the scanner start and end locations that are outside the image formation area. The scale factor is calculated based on the ratios of the times, the passing of the light beam is detected by the two optical detection units, to the reference time, the drawing frequency is changed according to the calculation result so as to accord with the reference time, so that the adjustment of the scale factor in the main scanning direction is carried out.
Moreover, in the color image forming apparatus of the types (3) and (4), the deviation of the scale factor between the respective optical paths directly causes the picture degradation such as the color deviation. When the optical element with the function to correct the position of the scanning line in the main scanning direction is made of a resin material with which the index of refraction of the optical element significantly changes by temperature, the two optical detection units mentioned above are indispensably needed.
To obviate the non-uniformity of the scanning speed in the main scanning direction of the item F above, the necessary scanning-speed uniformity is obtained with the precision including the thermal expansion of the optical scanner component parts in the image forming apparatus which does not need the positional precision so much as in the monochrome image forming apparatus of the type (1).
Moreover, in the color image forming apparatus which does not need the absolute position precision as that of the type (2), the same optical path is used for each color, and there is no difference of scanning-speed uniformity. Therefore the countermeasure that is essentially the same as in the image forming apparatus of the type (1) is taken.
Moreover, in the high precision plotter which needs the absolute position precision as in the types (1) and (2), and in the color image forming apparatus of the types (3) and (4), the different optical paths in the optical scanner are used for the respective colors. The drawing frequency is changed and adjusted within the time of the main scanning in such image forming apparatuses.
It is difficult to obtain the continuous change microscopically, and the undesired line image may be produced by the dot pitch irregularity in the main scanning direction.
When the optical element with the function to correct the position of the scanning line in the main scanning direction is made of a resin material with which the index of refraction of the optical element significantly changes by temperature, the scanning-speed uniformity of the main scanning direction is determined by the distribution of temperature within the optical element. In order to obviate the non-uniformity of the scanning speed in the main scanning direction, it is necessary to have many drawing cycle patterns with the optical scanner.
Thus, the adjustment techniques to obviate the positional deviations of the items A to E above are mostly established as described above. On the other hand, when the resin optical element is used in the optical scanner, it is necessary to carry out the adjustment for obviating the scanning-speed non-uniformity of the item F above with high precision, in order to avoid the picture degradation. However, in the current circumstances, the method for adjusting the scanning-speed non-uniformity with adequately high precision is not established yet, and it is difficult to avoid the picture degradation for the case of the optical scanner in which the resin optical element is used.